KVM: Prefixes segment functions that will be exported with "kvm_"

Prefixes functions that will be exported with kvm_.
We also prefixed set_segment() even if it still static
to be coherent.

signed-off-by: Guillaume Thouvenin <guillaume.thouvenin@ext.bull.net>
Signed-off-by: Laurent Vivier <laurent.vivier@bull.net>
Signed-off-by: Avi Kivity <avi@qumranet.com>